Among optical recording media, magnetooptical recording media are regarded promising because of their information capacity and great efforts are now concentrated on their research. The magnetooptical recording media generally include a recording layer in the form of a magnetic film formed on a transparent substrate through a dielectric layer. In more advanced form, a second dielectric layer is formed on the recording layer so that the recording layer intervenes between a pair of dielectric layers. Also a metal reflective layer is provided as the uppermost layer for increasing the output of reproduced signals.
Most promising among the metal reflective layers are those of aluminum and aluminum alloys from the standpoints of optical reflectivity and cost. For example, U.S. Pat. No. 4,717,628 discloses an Al--Ni alloy containing 2 to 10 at % of Ni which is advantageous in recording sensitivity and reproducing C/N. Also Japanese Patent Application Kokai (JP-A) Nos. 292753/1990, 285533/1990 and 267752/1990 disclose Al-Ta alloys, Al-Re alloys, and Al-Nb alloys, respectively.
Where such a metal reflective layer is provided, the second dielectric layer between the recording layer and the metal reflective layer is often of nitrides such as silicon nitride and aluminum nitride. In particular, silicon nitride having a refractive index of approximately 2 is believed to produce good enhancement effect when disposed below the metal reflective layer. However, both the metal reflective layer and the second dielectric layer of nitrides have sufficiently high thermal conductivity to promote heat dissipation from the recording layer during recording. This leads to several drawbacks including low recording sensitivity, and high recording power threshold (Pth) at which recording is started, which negates low power recording.
Optical recording requires not only a low recording power threshold, but also a low error rate and in the case of light modulation mode recording, for example, a low minimum recording power (Pmin) at which the byte error rate (BER) reaches 5.times.10.sup.-5 or less. The prior art structure, however, is also insufficient in this minimum recording power (Pmin).
On the other hand, the resolution between recording signals is reduced as the recording power is increased. In the light modulation mode, the recording power margin is defined as (P max-Pmin) wherein Pmax is the maximum recording power at which a certain resolution, for example, the resolution between 3T and 8T signals at a revolution number between 1,800 rpm and 3,600 rpm in accordance with ISO Standard, .sctn.324.1, decreases to 40%. For any recording mode, the recording power margin is desired to be as wide as possible. Every recording/reproducing apparatus has a number of variable factors in recording/reproducing conditions. More particularly, the recording laser power varies with every drive unit and optical and detection systems have their own factors. In addition, the apparatus experiences temperature variations and deterioration with time of laser power, deterioration with time of the optical and detection systems, tilt angle variations of the disk on loading, scattering due to dust, and the like. Interchangeability between drive units of different types is also necessary. For these reasons, a wider recording power margin is desired in order to always ensure stable recording/reproducing operation irrespective of varying factors in an apparatus or between different apparatus or between different types of apparatus. This facilitates the design and control of the drive unit. The prior art structure, however, has a narrow recording power margin.
Then for increasing the recording sensitivity and recording power margin of magnetooptical recording media for stable recording/reproducing operation, the inventors proposed the second dielectric layer formed on the metal reflective layer side of the recording layer that contains at least one of oxides of rare earth elements inclusive of Y, silicon oxide and silicon nitride (Japanese Patent Application No. 40277/1992). The use of such a second dielectric layer results in significantly increased recording sensitivity especially in the light modulation mode. More particularly, among the recording sensitivities, the recording power threshold (Pth) at which recording is started is significantly lowered. This threshold (Pth) lowering is believed due to the fact that the second dielectric layer has sufficiently low thermal conductivity to provide heat accumulating effect to the recording layer even in the presence of a metal reflective layer having high thermal conductivity. Moreover, this proposal also significantly lowers, among the recording sensitivities, the minimum recording power (Pmin) at which the byte error rate (BER) reaches 5.0.times.10.sup.-5 or less in the light modulation mode. And the recording power margin (Pmax-Pmin) between the maximum recording power Pmax at which a resolution of at least 40% is available and Pmin is substantially increased.
The recording methods for magnetooptical recording disks include light and magnetic field modulation modes. The light modulation mode performs recording using modulated recording laser light. In the magnetic field modulation mode, recording is carried out by directing recording laser light from an optical head to the recording layer of the disk in a DC manner for increasing the temperature thereof, and at the same time, applying a modulated magnetic field to the recording layer from a magnetic head disposed opposite to the optical head. Therefore, the latter mode enables over-writing recording and is thus in progress toward commercial application to rewritable disks as typified by mini-disks (MD).
In the magnetic field modulation mode, however, since continuous laser light is irradiated, heat is applied to the disk in a different manner from the light modulation mode, resulting in increased occurrence of jitter. The MD standard defines the optimum power as 1.4.times.Pmin and the recording power margin as (Pmax-Pmin)/2 wherein Pmin and Pmax are the minimum and maximum powers, respectively, at which the jitter of 3T signals is less than 40 nsec. when EFM signals are recorded at a circumferential linear velocity (CLV) of 1.4 m/s under an applied magnetic field of 200 Oe. As in the previous case, it is desired to have a lower optimum power and a wider margin.
It is, however, found that all the layer arrangements designed for a low optimum power and a wide recording power margin in recording of the light modulation mode do not provide an adequate optimum power and recording power margin when recorded in the magnetic field modulation mode.